The key to understanding the assertion that leaving the Catholic Church may put someone’s salvation at grave risk is to understand that Catholics believe that the Church was founded by Jesus Christ himself for our salvation. The Catholic Church is not just one optional Christian tradition among many possible valid choices; because it was founded by Christ, it is the Church to which everyone is called to belong, although some may be blamelessly ignorant of that fact in this life.

For someone to deliberately choose of his own free will to leave the Catholic Church even though he knows that Christ founded the Catholic Church and that he therefore has an obligation to belong to that Church, does indeed put that person at grave risk because leaving the Catholic Church constitutes grave matter. Full knowledge and full and free consent to grave matter constitutes mortal sin. If a mortal sin is unrepented before death, that person goes to hell.

Practically speaking, it is probable that many people who choose to leave the Catholic Church for another denomination are trying to follow God’s will as best they understand it. From an objective viewpoint, they are incorrect because God does not lead anyone out of his Church, but their desire and intent to follow his will to the best of their ability would mitigate their personal culpability for what is otherwise potentially a mortal sin.